It will be recalled that the diffusion bonding technique consists in bringing two surfaces of a given material into contact with each other at high temperature and under a certain pressure for a certain time. The two surfaces then bond together by atomic diffusion, this having the advantage of forming a bonded structure equivalent to the base structure of the material.
Of course, the quality of the bond depends on the operating parameters: temperature, pressure and time, but also parameters associated with the components to be joined together, generally in the form of plates: metallurgical structure and surface finish (cleanness, roughness). Consequently, it is paramount to eliminate any source of contamination of the surfaces to be joined together before they are heated to the temperature for the diffusion bonding step.
This surface cleaning operation is conventionally carried out by creating a vacuum in the cavity formed by the two surfaces to be bonded together. However, in the case of diffusion bonding associated with superplastic forming, a stop-off product is used to prevent diffusion bonding in those regions of the facing surfaces that will be subsequently be inflated in order to obtain a hollow mechanical part.
This type of product is composed of a binder, generally an organic binder, and a powder of a diffusion barrier material, such as a refractory material like a ceramic (for example yttrium oxide or alumina or boron nitride), or graphite.
After application of the stop-off product in a defined pattern corresponding to those areas of the surfaces that are not to be joined together by diffusion bonding, the binder is degraded so as to conserve only the powder of the stop-off product, which is formed from particles that exhibit anti-adhesion properties, preventing the atoms of the materials of the plates to be bonded from diffusing.
During this degradation of the binder by raising the temperature to generally between 200 and 400° C., residues, particularly gas residues, form, which may contaminate, to various degrees depending on their chemical composition, the surfaces to be bonded.
Thus, it will be understood that is essential also to remove the degradation residues of the stop-off product.
It should be noted that the mechanical strength of the stop-off product is greatly reduced after this degradation, so that it is necessary to avoid manipulating and/or moving the parts to be bonded or to create disturbances within the cavity that could cause the particles of the stop-off powder to be disseminated in those regions of the surfaces to be bonded that have to be joined by diffusion bonding.
Conventionally, these degradation residues are reduced by creating a partial vacuum in the cavity formed by the two surfaces to be bonded.
It has also been proposed to circulate an inert gas, such as argon, in this cavity and then to create a partial vacuum in this cavity for the actual diffusion bonding step (FR 2 754 478).
This solution is relatively difficult to implement since it is necessary to install a system of pipes and fittings for circulation of the inert gas and for the subsequent vacuum created, this system also constituting a source of possible leaks and making the diffusion bonding operation more complex to implement. Furthermore, this solution allows only a single part to be treated at a time and the creation of a vacuum lengthens the manufacturing time.
In addition, it should be noted that when the binders volatilize the mechanical behaviour of the stop-off product greatly deteriorates in such a way that the flow of inert gas circulating in the cavity may result in local dissemination of the stop-off particles on the surfaces to be bonded.
Finally, it should be noted that despite the creation of a partial vacuum in the cavity at the end of degradation, the risk of retaining inert gas, trapped in certain regions of the cavity, is not zero, it being possible for such residual gas pockets to locally prevent diffusion bonding.
Moreover, U.S. Pat. No. 5,484,977 and U.S. Pat. No. 5,273,202 have proposed the elimination of the residues from the degradation of the binder for the stop-off products by the fact that the assembly is placed in a chamber under a dynamic partial vacuum, that is to say with continuous suction in order to extract the gases coming from the degradation of the binder.
Here again, it is difficult to treat several parts at the same time because of the volumes to be put under a partial vacuum, the treatment time is long, and it is necessary to install a large vacuum chamber. This makes the process expensive to implement.